1. Field
The disclosure relates to an improved process for converting synthesis gas to liquid hydrocarbon mixtures useful in the production of fuels and petrochemicals by contacting the synthesis gas with multiple catalysts in alternating layers within a single reactor tube.
2. Description of Related Art
The majority of combustible liquid fuels used in the world today are derived from crude oil. However, there are several limitations to using crude oil as a fuel source. For example, crude oil is in limited supply.
Alternative sources for developing combustible liquid fuel are desirable. An abundant resource is natural gas. The conversion of natural gas to combustible liquid fuel typically involves a first step of converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. Fischer-Tropsch synthesis is a known means for converting syngas to higher molecular weight hydrocarbon products. Fischer-Tropsch products include naphtha which can be further upgraded to produce gasoline or liquefied petroleum gas (LPG). Fischer-Tropsch naphtha can also be blended with diesel fuel to modify low temperature properties. Fischer-Tropsch naphtha can also be converted to light olefins, such as, for example, ethylene. Olefins from Fischer-Tropsch naphtha can be converted to diesel, or further processed to form plastics. Fischer-Tropsch synthesis is often performed under conditions which produce a large quantity of C21+ wax, also referred to as “Fischer-Tropsch wax,” which must be hydroprocessed to provide distillate fuels. Often, the wax is hydrocracked to reduce the chain length, and then hydrotreated to reduce oxygenates and olefins to paraffins. Hydrocracking tends to reduce the chain length of all of the hydrocarbons in the feed. When the feed includes hydrocarbons that are already in a desired range, hydrocracking of these hydrocarbons is undesirable.
Considerably different process conditions are generally used for Fischer-Tropsch synthesis and for hydrocracking of Fischer-Tropsch wax using relatively acidic catalysts such as, for example, SSZ-32 or ZSM-5. For this reason commercial Fischer-Tropsch plants utilize separate reactors for the Fischer-Tropsch synthesis and for the subsequent hydrocracking of the product wax, and complicated and expensive separation schemes may be used to separate solid wax from lighter products.
Because the Fischer-Tropsch reaction is highly exothermic, the problem of how to remove reaction heat is a primary challenge in designing and operating Fischer-Tropsch reactors. Fixed bed reactors typically contain many narrow reactor tubes placed within a cooling medium to remove heat. Heat is also managed by practices such as running the reactor at a high gas flow rate and recycling a portion of the tail gas.
It would be advantageous to provide an economic process in which both synthesis gas conversion and product hydrocracking are combined within a single reactor at a common set of conditions resulting in a high yield of naphtha range liquid hydrocarbons and a low yield of wax. It would further be advantageous to provide such a process with improved heat management within the reactor.